Structure of RdxA - an oxygen-insensitive nitroreductase essential for metronidazole activation in Helicobacter pylori

The RdxA oxygen-insensitive nitroreductase of the human gastric pathogen Helicobacter similar to pylori is responsible for the susceptibility of this organism to the redox active prodrug metronidazole [2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol]. Loss-of-function mutations in rdxA are primarily responsible for resistance to this therapeutic. RdxA exhibits potent NADPH oxidase activity under aerobic conditions and metronidazole reductase activity under strictly anaerobic conditions. In the present study, we report the crystal structure of RdxA, which is a homodimer exhibiting domain swapping and containing two molecules of FMN bound at the dimer interface. We have found a gap between the side chain of Tyr47 and the isoalloxazine ring of FMN that appears to be appropriate for substrate binding. The structure does not include residues 97128, which correspond to a locally unstable part of the NTR from Escherichia similar to coli, and might be involved in cofactor binding. Comparison of H.similar to pylori RdxA with other oxidoreductases of known structure suggests that RdxA may belong to a new subgroup of oxidoreductases in which a cysteine side chain close to the FMN cofactor could be involved in the reductive activity. In this respect, the mutation of C159 to A or S (C159A/S) has resulted in a loss of metronidazole reductase activity but not NADPH oxidase activity. The RdxA structure enables the interpretation of the many loss-of-function mutations described previously, including those affecting C159, a residue whose interaction with FMN is required for the nitroreduction of metronidazole. The present studies provide unique insights into the redox behaviour of the flavin in this key enzyme for metronidazole activation, including a potential use in gene therapy.